CAIRO (AP) — A truck bomb struck the main security headquarters in Cairo on Friday, one of a string of bombings targeting police within a 10-hour period, killing six people. The most significant attack yet in the Egyptian capital fueled a furious backlash against the Muslim Brotherhood amid rising fears of a militant insurgency.

The mayhem on the eve of the third anniversary of Egypt's once-hopeful revolution pointed to the dangerous slide Egypt has taken since last summer's military ouster of Islamist President Mohammed Morsi: A mounting confrontation between the military-backed government and Islamist opponents amid the escalating violence.

In the hours after the blast, angry residents — some chanting for the "execution" of Brotherhood members — joined police in clashes with the group's supporters holding their daily street protests against the government. Smoke rose over Cairo from fires, and fighting around the country left 14 more people dead.

Saturday, the anniversary of the start of the 18-day uprising that ousted autocrat Hosni Mubarak, raised the potential for new violence, as both military supporters and the Islamists vowed to take to the streets with rival rallies.

After Friday's blasts, interim President Adli Mansour vowed to "uproot terrorism," just as the government crushed a militant insurgency in the 1990s. The state "will not show them pity or mercy," he said. "We ... will not hesitate to take the necessary measures."

That could spell an escalation in the crackdown that the government has waged against Morsi's Muslim Brotherhood since his July 3 ouster.

Thousands of Islamists have already been arrested and hundreds killed, with authorities accusing the group of being behind militant violence. The Brotherhood, which allied with some radical groups while in power, denies the claim, saying the government is using it to justify its drive to eliminate it as a rival. The crackdown has expanded to silence other forms of dissent, with arrests of secular activists critical of the military, security forces and the new administration.

For activists, that has raised deep concerns over a return of a police state despite the government's promises of democracy.

But among a broad swath of the public, those concerns are eclipsed by fear of the wave of militant bombings and shootings since the coup, which have largely targeted police but increasingly hit in public areas taking civilian casualties. And the public fury has been funneled at the Brotherhood: After Friday's bombings, TV stations aired telephone calls from viewers pleading with army chief Gen. Abdel-Fattah el-Sissi to definitively crush the group.

"Execution for Morsi and his leaders!" one man shouted through a megaphone as an angry crowd gathered outside the Cairo security headquarters, hit in Friday's first bombing.

"Morsi is the butcher and el-Sissi will slaughter him!" screamed a woman, holding up a picture depicting the Brotherhood as sheep.

The day's violence began with the 6:30 a.m. blast at the security headquarters, located on downtown Bab el-Khalq Square. Security camera footage showed a white pick-up truck pulling up to the building's gate. A man jumped out of it, got into another car and drove off. Two policemen inspected the truck for a moment before returning to the headquarters. Two minutes later, it exploded.

The powerful blast ripped down a main avenue, knocking out shop windows for more than 500 meters (yards). The eight-story headquarters' facade was shattered, with air conditioning units left dangling out of broken windows, and a 6-foot crater was blasted into the pavement.

The explosion also wrecked Cairo's renowned Islamic Arts Museum, directly across the street, blasting out its windows, causing ceilings to collapse, smashing display cases of porcelain and glasswork and breaking water pipes that sprayed over manuscripts. Museum experts said key pieces in its collection of Islamic artifacts were damaged.

Abdullah el-Sayyed, a 26-year-old salesman who lives behind the headquarters, said he was wakened by the blast, followed by heavy gunfire by frantic policemen. "They were firing their guns in panic as if to call for rescue," he said.

He said he plans to return to his home village in Fayoum, south of Cairo, because he no longer feels safe. "It's not worth it anymore to stay here. Every day I ride the metro and go past here," he said.

After the blast, several police officers sat on the sidewalk outside the building, weeping as ambulances rushed in. A body lay on the ground, covered by a sheet as a crowd of distraught-looking residents surveyed the damage.

Touring the site, Interior Minister Mohammed Ibrahim, who is in charge of police, called the bombings a "vile terrorist act" and implicitly blamed the Brotherhood, without naming it. "They will reach a point where coexistence will be impossible," he said.

Security officials later said three suspects had been identified in the security headquarters attack, adding that they belonged to the Brotherhood and "extremist groups." The officials spoke on condition of anonymity because they were not authorized to release the information.

About two hours after the Bab el-Khalq blast, attackers threw a bomb at a police car near a metro station in the Dokki district on the other side of the Nile River, killing one person and wounding eight others, the prosecutors' office said.

A third, smaller blast targeted the Talbiya police station about four kilometers (two miles) from the famed Giza Pyramids but caused no casualties, security officials said. They spoke on condition of anonymity because they were not authorized to speak to the media.

Hours after the attacks, the Brotherhood held its daily protests that they have vowed to step up ahead of Saturday's anniversary. The marches quickly turned into clashes with police, joined by residents furious at the Brotherhood, in several districts of Cairo and in cities across the country. The Health Ministry said 14 people were killed in the violence.

In one Cairo neighborhood, pro-Morsi protesters clashing with security forces set fire to a police kiosk, sending a pall of smoke in the air. In the Mediterranean city of Alexandria, residents throwing stones and firing rounds of birdshot killed one Brotherhood supporter when they attacked Islamists marching after the funeral of a student protester killed a day earlier.

As police drove back from clashes with Brotherhood supporters in the capital's Giza district, they were hit by the day's fourth bombing — a roadside explosive that killed one person and wounded four others.

Police and soldiers have been targeted by multiple attacks in recent months. In December, a suicide car bombing blasted the main security headquarters of a Nile Delta province, killing 16 people. The interior minister survived an attempted car-bombing assassination attempt in Cairo in September.

An al-Qaida-inspired group, Ansar Beit al-Maqdis, or the Champions of Jerusalem, has claimed responsibility for most attacks, saying they aimed to avenge the killings of Morsi's supporters in the post-coup crackdown. On Thursday, the group issued an online audio statement warning police and soldiers to defect or face new attacks.

The Islamist alliance grouping the Brotherhood and its allies condemned Friday's attacks and blamed them on the Interior Ministry, saying it wanted to turn the public against the Islamists.

It vowed to push ahead with protests Saturday, saying, "the revolution will continue down its peaceful track to bring down the military coup."

Government supporters are also planning giant rallies on Saturday to show their backing for the military — and to call for army chief el-Sissi to run for president.